text: an informal one:
  
  
    Please, don't forget to reply back my postcard and letter
  
  
  拜託．唔好唔記得回信
  
  
    I'm looking forward to get soon news from you
  
  
  我想早啲收到你嘅消息
  
  
    Best regards, wish you good luck
  
  
  祝安康
  
  about a formal version, well, it's tricky. because, there're "canned" terms for correspondences in chinese, and these terms are in literary chinese.
  
  if you google "尺牘用語", there're numerous pages. briefly, you must know who has superiority, you, or the receiver; then choose the appropriate opening &amp;  ending.
  
  即覆為盼 - hope for your immediate reply
  
  敬請臺安 - for equal status, roughly "wish you calm"
  
  personally, i "write" colloquial cantonese, mixed with english words in email with my friends. these literary chinese terms are simply, "overkill" in context of cantonese.
  
  edited, if the receiver is more important, only use the two below, previous suggestions are not appropriate.
  
  祈請示覆 ~ pray for your reply
  
  祈 kei4　請 ching2　示 si6　覆 fuk1
  
  敬請鈞安 ~ wish you calm
  
  敬 ging3　請 ching2　鈞 gwan1　安 on1
  
  you may click the word to play the sound file :)
  
  the dilemma is, these terms are not colloquial cantonese. i doubt that people would understand, and guess the correct characters when they hear your saying.
  
  c'est la vie, the written chinese is different from the spoken chinese.
  
  anyway, don't misuse "canned" terms for equal, or lower status; it's extremely rude.
  
  last, you may use the 漢語多功能字庫 for learning cantonese pronunciation:
  
  http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/
  
  or, if you're using mac os x, the text to speech function is quite good, have a try :)